The resistors are made of an electrically resistive film material which is applied in form of a trace onto an insulating substrate. The two ends of the trace of the second resistor overlap at least in part with a first and a second contacting terminal, respectively, and the two ends of the trace of the first resistor overlap at least in part with the second and a third contacting terminal, respectively. Typically, the second contacting terminal is connected to a reference potential such as ground, the third contacting terminal is connected to an input voltage potential, and the first contacting terminal provides, with respect to the reference potential, a voltage which is proportional to the voltage ratio and the input voltage potential. The divider's voltage ratio has a value between one hundred and one million
The voltage divider may consist in its simplest form of just two serial resistors, one with a high resistance value and the other with a low resistance value. In more advanced cases, the one or both of the serial resistors can be replaced by resistive networks having respective equivalent resistance values. These resistors or corresponding resistor networks may in the following also be called high and low ohmic resistors, respectively. In the voltage divider, both the high and low ohmic resistors are brought onto the same substrate.
Different techniques are known to manufacture resistors by bringing a noninsulating electrically resistive film or foil material, such as metal film or metal foil, e.g. nickel chromium, cermet film, e.g. tantalum nitride, ruthenium dioxide, bismuth ruthenate, carbon film, or a film of a composite material based on a mixture of glass and cermet onto an insulating substrate. In rare cases, the electrically resistive film material may consist of multiple layers of different of the above named materials. The insulating substrate can be ceramic, silicon, glass or some other synthetic material, and the film material is applied to the substrate by methods such as sputtering (thin film), screen and stencil printing (thick film) or direct printing through a nozzle (thick film). The insulating substrate may have the form of a flat planar sheet or of a cylinder, and accordingly the resistive film is deposited either onto a two-dimensional planar surface or onto a three-dimensional axially symmetric surface. In the voltage divider, both the high and low ohmic resistors are brought onto the same substrate. In addition, highly conductive structures with considerable lower resistivity than the film material of the resistors are deposited on the substrate as well. The highly conductive structures are intended to be used as contacting terminals, and they are placed on the substrate in such a way that the resistive film material of the resistors overlaps at least partly with them.
In order to achieve voltage ratios of significantly more than unity and at the same time to reduce the size of the voltage divider, it is known to arrange the resistive film material of the high ohmic resistor in a long and narrow trace, where the trace is shaped like a meandering form. The term meandering form means that the trace is not just a straight line but curved in such a way that a long length is achieved on a small substrate area. The meandering form may look for example like a square wave, a triangle wave, a sine wave, a serpentine, a zigzag or—in the three-dimensional case—a helical form. This is for example described in U.S. Pat. No. 5,521,576 for thick film resistors and in U.S. Pat. No. 7,079,004 B2 for thin film AC voltage dividers. As is disclosed there as well, the low resistance value of the low ohmic resistor is commonly obtained by arranging the resistive film material in a short and wide trace.
In general, the above described resistive voltage dividers can be used for a wide range of voltage levels, from low over medium up to high voltage applications. While the present disclosure originates from the area of medium voltage sensors, such as the KEVCD and KEVA sensor types by ABB, which are commonly applicable to a voltage range between 3.6 kV and 36 kV, its area of application is not limited to this voltage range.
As is known from the art, for example from U.S. Pat. Nos. 5,521,576 and 7,079,004 B2, the first contacting terminal is commonly placed between the second and the third contacting terminals. The second contacting terminal is connected to a reference potential such as ground, the third contacting terminal is connected to an input voltage potential, and the first contacting terminal provides, with respect to the reference potential, a voltage which is proportional to the voltage ratio and the input voltage potential. As a result, a first parasitic capacitance occurs in parallel with the high ohmic resistor, i.e. between the first and the third contacting terminals, and a second parasitic capacitance occurs in parallel with the low ohmic resistor, i.e. between the first and the second contacting terminals. It is assumed that the high ohmic resistor has a resistance value R1, the low ohmic resistor a resistance value R2, the first parasitic capacitance a value C1 and the second parasitic capacitance a value C2. When an AC voltage is applied to the input of the voltage divider between the second and the third contacting terminals, the AC output voltage between the second and the first contacting terminals shows no phase error in case that the voltage ratio (R1+R2)/R2 equals the ratio of the second parasitic capacitance to the first parasitic capacitance (C1+C2)/C1. The phase error increases with an increasing mismatch of these ratios.
The inventors have understood that for voltage dividers with a high voltage ratio (R1+R2)/R2 above one hundred, the corresponding resistance ratio (R1+R2)/R2 is significantly higher than the ratio of the parasitic capacitances (C1+C2)/C1, which leads to a high phase error. The phase error can be corrected by adding compensation capacitors in parallel to the low ohmic resistor. However, this adds to the complexity of the voltage divider, which increases the designing and manufacturing effort and thereby the costs. In addition, the compensation of the phase error is usually not achieved over the whole operating temperature range, humidity range and life time of the voltage divider, since the parasitic capacitances and the compensation capacitors have different temperature coefficients, humidity absorption properties and long term drifting. In other words, the phase accuracy can not be sufficiently ensured when using compensation capacitors.